A "telecine" is an apparatus utilized to transfer a motion picture film, for example a negative film shot in a motion picture camera, to a video signal format for storage or broadcast. Telecines are employed by motion picture and television production and post-production facilities to create video tapes of movies, commercials, news clips, and the like.
In a typical telecine, a source of light such as a flying spot scanner or a laser beam scans the film, or a light bulb projects the image onto a light detecting array, frame by frame, in a line-by-line or raster scan fashion. The light from the light source is transmitted through the film in a "film gate" on the telecine, and received by a light detector positioned to receive light transmitted through the film. A film gate includes an aperture ("frame aperture") for passing light from the light source to the telecine light detector and supports the film as it is scanned. The transmitted light is then converted into electrical signals, converted into an appropriate video signal format such as NTSC or PAL, and stored on a video tape or other storage device.
A CCD line array telecine is an apparatus in which consecutive frames in a cinema or movie film are optically scanned in a raster fashion by scanning each frame, one line at a time, with an imaging array of charge coupled devices, wherein the picture information is thereby converted by the CCD array into electrical signals. From the electrical signals, a television signal is ultimately derived from scanning respective consecutive film frames.
There are two primary types of film transfer in telecines--(1) intermittent or "start/stop", and (2) real-time or continuous motion. U.S. Pat. No. 5,266,979 to Brown et al. is an example of the latter type of device. In the intermittent or "start/stop" type of film transfer, each frame of the film is located or "registered" at the frame aperture; in some cases, the film is held motionless during the scanning process; in other cases, a frame is pulled past a frame aperture slit that allows light to pass through a "line" in the film onto the CCD imaging array in the telecine. In the real-time or continuous motion telecine, the film is moved continuously with a film drive to move the film through the machine during the scanning process.
Servomotors and related control circuitry are used in the prior art in an effort to move the film at a constant rate relative to the imaging array, but are considered inadequate to provide highly-reliable speed regulation for that purpose. If the speed of the film varies as the frame is moved past the aperture slit, distortions in the scanned image are created.
A particular difficulty encountered in telecine devices is the maintenance of the image on the film in reference to a stable image reference. The problem is particularly pronounced when images from different sources are superimposed, for example, when a computer-generated title or other graphics are superimposed against a scene represented by a motion picture film. If the image on the film is not stable relative to the superimposed graphics such as a title, the title may be seen to weave or jitter in relationship to the background. The jitter or weave problem derives from the fact that the positional reference for the film and for the overlaid graphics are not the same. When a film is originally shot in a camera, the image is mechanically registered on a particular film frame relative to the sprocket holes in the film. The tines of a sprocket in the camera engage the sprocket holes in the film to register the film relative to the camera's optics. Such devices, including cameras, are considered mechanically or machine pin registered.
When a film from a motion picture camera is then to be transferred to video in a telecine, the film may not necessarily be machine pin registered but drawn through the machine by a continuous motion film drive. Typically, a drive capstan pulls the film through the film gate in a continuous motion device. In some continuous motion telecines, the film is edge-guided during the transfer. Film weave or jitter may be introduced as the edge of the film varies relative to the position of the sprocket hole. It is known that film edges tend to vary in distance from the sprocket holes due to manufacturing imperfections and tolerances in the film.
Various approaches have been employed to minimize film weave in film-to-tape transfers. One approach involves use of a single mechanical sprocket drive for moving the film through the telecine. This method introduces small but pronounced speed changes as the sprocket tines enter the sprocket holes, and therefore introduces another source of jitter. Moreover, mechanical sprocket drives are not favored because of the wear and tear on the film.
Other mechanical solutions involve mechanical pin registration with the start/stop devices discussed above. In a conventional start/stop approach, the film sprocket holes are lowered, frame by frame, onto stationary register pins with a stepping motor controlled by an auxiliary computer. The film is held steady by pins during the transfer, and then a gate attachment is released to allow the film to advance.
Such start/stop mechanical pin registration devices are complicated mechanically, limited in speed, and produce significant wear and tear on the film because of repeated acceleration and deceleration of the film. Nonetheless, there are certain desirable applications for the start/stop approach. In particular, if a film frame can be drawn past the scanning aperture in a sufficiently precise motion, distortion in the picture (typically in the vertical dimension, or "line distortion") can be substantially reduced or eliminated.
Prior to the present invention, it has not been possible to transport a single frame in a start/stop motion with extremely precise motion.
One known approach to the problem of film weave and jitter relies upon electronic scanning of the sprocket holes and generation of correction signals used in the raster scanning process. U.S. Pat. Nos. 4,823,204 and 4,104,680 to Holland are directed to electronic methods and apparatus for correction of film weave, by scanning the sprocket hole of the film and generating a correction signal to compensate for the film weave. The film weave correction signal is then used to adjust the position of the scanning beam to compensate for motion of the film as detected by the scanning of the sprocket holes. However, systems that use the sprocket hole scanning approach are electronically complex and expensive.
Electronic methods for film weave correction that involve scanning of the sprocket holes such as shown in U.S. Pat. Nos. 4,823,204 and 4,104,680 typically involve modifying the film gate of a telecine such that the sprocket hole area of the film may be scanned. Thus, a hole or slot must be machined in the film gate to permit the detecting beam to scan the sprocket hole. Additionally, it may be necessary to supply higher powered deflection amplifiers for the rapid deflection of the scanning beam of the light source to the sprocket hole area, or provision of a separate detecting beam, both of which add to the complexity and expense of these electronic methods for film weave correction.
The conventional film gate used in a prior art Rank telecine for 35 mm film includes a single feed sprocket. The teeth on this sprocket are very small relative to the dimensions of the film perforations and consequently does not prevent the weave or jitter of the film. In addition, the Rank mechanism only includes the single sprocket immediately preceding the frame aperture and thence to a roller without any teeth or other guides for imparting stability. There is consequently no control of film movement either going into or leaving the film gate.
Accordingly, there is a need for an improved stop motion film gate that is able to transport a film frame past a scanning aperture in a precisely controlled and constant linear motion, to reduce distortions in the scanned image produced by nonlinearities in the transport motion.